It is known to produce multilayered structures to take advantage of different properties exhibited by the various layers in the structures. Typical of multilayered structures are multilayer films in which different layers have specific characteristics. These films may then be used in packaging applications having customized properties that may be required for a particular packaged product. For example, films which are used to protect food, medicines, paints, adhesives, biomaterials, chemicals, etc., require properties such as good thermal and environmental stability, UV blocking characteristics, transparency in visible light wavelengths and excellent moisture barrier protection.
In forming multilayer films, it is often desirable to obtain measurements of the basis weight and/or thickness of the individual layers. The “basis weight” is the mass per unit of area of a material, typically measured in grams per square meter (gsm). Various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. These systems include conventional infrared (IR) absorption techniques whereby radiation of the appropriate wavelength(s) and bandwidth(s) is selected to measure the polymer(s). In the case where the film consists of two layers each formed of a different polymer, the selected radiation has at least two separate wavelength regions, one region being preferentially absorbed by polymers of the first layer and a second region being preferentially absorbed by polymers of the second layer. The thickness of each layer is proportional to the amount of radiation absorbed. As is apparent, this method is applicable when each layer is made of a different polymer; it is much less useful when one or more layers consist of polymer blends as the spectrum from each polymer typically overlaps with spectra from other polymers present in a multilayer structure. With the myriad of polymers employed in multilayer films, it is extremely difficult to find a specific region, in the radiation spectra and especially in the near infrared region, for each of the particular polymers that are substantially free from interference from other polymers. As is also apparent, the technique is not applicable where a particular polymer is present in more than one layer. Finally, conventional absorption techniques are not accurate when the layers are very thin at the gsm level.
Another system for measuring plastic films employs beta gauges that are nuclear measuring devices which emit beta rays, and which have been used to measure characteristics of single-layer compositions, such as single-layer plastic films. Typically, a single-layer composition has a known beta ray absorption coefficient and a known density. Using a beta gauge, the basis weight of the single-layer plastic film can be determined. That is, by using the Beer-Lambert law, the relatively accurate, raw output of the beta gauge and the known absorption coefficient of the single-layer plastic film, the mass per unit area can be calculated for each point on the film to be measured. Similarly, the thickness of the single-layer plastic film may be calculated at each such point. The thickness of a point on the film corresponds to the mass per unit area of that point, as previously determined, divided by the known density of the plastic used to form the film. Alternatively, the mass per unit area and/or thickness of a single-layer plastic film can be measured by using a calibrated beta gauge. Calibration directly relates the beta absorption to the physical properties of the product to be measured.
Since beta gauges can only measure total film thickness, a number of beta gauges is needed to measure multilayer products. The individual layer thicknesses are obtained by measuring the total thickness of the product before and after each layer is applied. The individual thicknesses are given by subtracting the total thickness measured at the two locations. This works in principle, however the requirement of more than one scanner and sensor leads to additional costs. It is also often not possible to perform measurements before and after a layer is applied as in the case of co-extruded multilayer plastic films.